The present invention relates generally to pressurized water reactor steam generators, and more particularly to a method for controlling crevice chemistry on the secondary side of a pressurized water reactor steam generator.
In the steam generator of a pressurized water reactor, it is important to maintain proper secondary side water chemistry to provide for long-term integrity of the steam generator components. Maintenance of proper water chemistry is not necessarily straightforward, because when evaporation occurs on the secondary side of the steam generator, impurities concentrate in regions of restricted flow, designated hereinafter as crevices, resulting in concentrated aqueous solutions which can be corrosive toward steam-generator materials of construction. These impurities can be introduced as a result of weld repair, plant modification, component replacement, leakage in the condenser, or makeup water contamination. These concentrated impurity solutions are known to be corrosive even though the specific corrosive specie has not been identified. It is known, however, that acidic or alkaline conditions, typically produced in these concentrated solutions, accelerate corrosion. For this reason, it is beneficial to maintain neutral (i.e. neither acidic nor alkaline) conditions for these concentrated solutions of impurities. In this way, the rate of corrosion of various components such as those made of Alloy 600 can be minimized.
To minimize corrosion and thereby provide for long-term use of steam generators in pressurized water reactors, computer programs have been developed to assist in maintaining acceptable water chemistry standards. Emphasis has been placed on "ratio control," which is a procedure through which an effort is made to maintain a constant, neutral, ratio of cations to anions on the secondary side of the steam generator.
As traditionally practiced, "ratio control" involves the measurement, recording, and adjustment of one of the following concentration ratios for the bulk water chemistry during operation of the steam generator: EQU [Na.sup.+ ]/[Cl.sup.- ].about.0.5 EQU [Na.sup.+ ]/{0.7[Cl.sup.- ]+2[SO.sub.4.sup.2- ]}.about.1,
in which concentrations are expressed in terms of moles or micromoles per kilogram of water. Details of this procedure are provided in PWR Secondary Water Chemistry Guidelines--Revision 3, TR - 102134, Electric Power Research Institute, May 1993, Appendix D (EPRI Guidelines).
It is known that there are differences between cation-to-anion ratios in the bulk water of a steam generator, and ratios in the crevices. More specifically, the sodium-to-chloride ratio in the bulk water is likely to be lower than the sodium-to-chloride ratio deep within crevices within the steam generator, because sodium hides out more effectively than chloride. A practical way to allow for these differing hideout efficiencies is to maintain a bulk-water molar ratio of sodium to chloride less than 1. (In the absence of other information, the guidelines document, TR-102134, suggests beginning with a ratio of 0.5.) The intent is that such a bulk-water ratio should maintain the molar ratio within crevices near 1. The thinking has been that, within crevices, a ratio of sodium to chloride near 1 would keep the crevice pH near neutral.
A term which distinguishes the chemical properties deep within crevices, from bulk-water properties, and which will be used in this document, is "crevice chemistry." As indicated in the EPRI Guidelines, the crevice chemistry on the secondary side of a steam generator can be predicted or estimated based upon "hideout return data," which is a measurement of the secondary-water dissolved impurity concentrations which result from plant shut-down. Hideout return data can be collected during temporary plant shut-down and can then be used as a basis for modifying bulk water chemistry during subsequent periods of operation.
It is known that the impurities present in steam-generator bulk water become increasingly concentrated in steam generator crevices over time. To predict these concentrations, the PWR industry routinely has used the computer code, MULTEQ, which was developed for the purpose of simulating these changes in concentration. As indicated in the EPRI Guidelines, MULTEQ is an interactive FORTRAN computer program which predicts changes in pH and solution concentration, based upon initial concentration data, as the impurities in the water become increasingly concentrated as a result of evaporation. The program takes into account equilibrium relations, including particular combinations of components, precipitation reactions, and volatilization. Based upon the characteristics of a particular system, MULTEQ calculates concentration variations in the liquid phase as boiling proceeds.
MULTEQ has three program options, which correspond to three different ways in which the solution may be concentrated. The first and second program options assume that a closed (static) system is involved, in which the ratio of the mass of water in the liquid phase is varied while the total mass of the stream remains constant. The first option is recommended for use in modeling tube-to-tube support plate crevices. In this type of system, the precipitates remain in equilibrium with the liquid and vapor phases. The second program option generally is used to model tubesheet crevices or very restricted tube support plate crevices. In this second type of system, precipitates are removed from the system as they form. As a result, the precipitates have no subsequent effect on liquid or vapor phase chemistry. The third option is useful to model the accumulation of chemicals in sludge piles or scales. In accordance with this system, the mass of liquid water flowing into the system is equated to the mass of water vapor flowing out of the system.
Typically, the MULTEQ computer system is used in the following manner. Concentration data is input for concentrations of sodium, calcium, chloride, silicate, sulfate, and, optionally, additional parameters, such as magnesium. Based upon the input concentrations, the MULTEQ code will predict increases in solution concentrations to a predetermined end point. The resulting calculations can be expressed in terms of a plot of pH (at the operating temperature) versus ionic strength. The drawback of the MULTEQ system is that MULTEQ will simulate increases in concentration only after ion concentrations have been input. In order to obtain predictive output, it is necessary to make adjustments to the input concentrations, such as the ratio of sodium to chloride, and then run the computer program and subsequently observe the result. Thus, this system does not provide for an up-front indication of specific changes to be made in sodium and/or chloride concentrations in order to arrive at a desired result.